Although magnetic disk subsystems are widely employed as a secondary storage device for a computer, their I/O performance is always lower than the I/O performance of a main storage by three to four orders of magnitude. Thus, efforts have long been made to reduce this difference. As a method of improving the I/O performance of the subsystem, there is known a disk array system in which a plurality of magnetic disk drives are operated in parallel to distribute data for storage in these plurality of magnetic disk drives.
In the disk array system, by operating a plurality of the disk drives in parallel, the I/O performance and the throughput performance of the system during sequential access can be improved. However, it is necessary to perform data transfers concurrently with a plurality of the magnetic disk drives. Thus, in order to improve the performance of the system, the high data transfer performance is required for the internal paths of the disk array control device as well, which connects the disk drives and the host I/F units.
FIG. 1 is a conceptual diagram showing a disk array system including a disk array control device, which shows a related art according to the present invention. A disk array control device 200 in FIG. 1 for reference comprises host interface units 220, disk interface units 230, and shared memory units 240, all of which are interconnected by a common bus 210. A host interface unit 220 comprises a channel I/F circuit 120 and a processor 110, and controls exchange of data with a host computer 300. A disk interface unit 230 comprises a disk I/F circuit 150 and a processor 140, and controls exchange of data with a plurality of disk drives 500. A shared memory unit 240 temporarily stores control information for the system and data to be written into the disk drives. (A disk array having such a configuration will be provisionally referred to as a shared-bus-connected disk array.)
Generally, in this configuration of the shared-bus-connected disk array, a throughput of the common bus 210 becomes a bottleneck of a system performance. In most cases, the throughput of the bus is determined by its data bus width and operating frequency. In the case of the common bus, both the bus width and the operating frequency are almost approaching their limits in recent years, so that there has been no improvement in the system performance of the shared-bus-connected disk array.
In order to overcome this problem, JP-A-10-333836 discloses a disk array having a configuration where a plurality of interface units connected to host computers and interface units connected to a plurality of magnetic disk drives are connected to a plurality of shared memory units through an interconnection network using a switch. The respective host interface units and the respective magnetic disk interface units make access to a shared memory unit by way of the switch of the interconnection network. (A disk array of such a configuration will be provisionally referred to as a starnet connection disk array.)
Since the physical number of connection paths between the shared memory units and the switch (memory connection paths) is greater than the physical number of paths between the shared memory units and the common bus for the shared-bus-connected disk array, the number of connection paths per starnet connection disk array is extremely greater than the number of connection paths per shared-bus-connected disk array, though the transfer throughput of a single connection path in the starnet connection disk array cannot be increased as compared with the transfer throughput of a single common bus due to the constraints of the physical number of pins. Accordingly, the threshold transfer throughput of the entire starnet disk array system is large, thus allowing improvement in the system performance. Since the number of the connection paths is increased by addition of the host interface units, magnetic disk interface units, or shared memory units, the system performance is improved in proportion to addition of functional units. In this sense, the starnet connection disk array would be a scalable architecture.
With a rapid improvement in the performance of a host computer and a faster network, market requirements for the performance of the disk array system have been much more increased. On contrast therewith, in the shared-bus-connected disk array, an improvement in the system performance is becoming difficult due to the bottleneck in the performance of the common bus, so that the shared-bus-connected disk array cannot satisfy the market requirements. On the other hand, in the starnet connection disk array, as described above, by increasing the number of the connection paths, an improvement in the throughput performance of the system would be possible.
However, even in the starnet connection disk array, the number of the connection paths is actually limited due to the constraints of installation. Therefore, just by increasing the number of the connection paths, a further improvement in the performance of the system is not expected. Further, even if paths for one-to-one connection are employed between the respective interface units and the switch, and between the switch and the respective shared memory units, there exists a limit to the operating frequency of the paths. Thus, an improvement in the performance of a single connection path is also limited. Accordingly, it becomes necessary to improve the throughput of the internal paths in the system, on condition that the number of the connection paths is limited.
The starnet connection disk array aims at improvement in the throughput of the internal paths in the entire system, by simultaneous operation of a plurality of connection paths. In disk arrays in general, access requests from a plurality of the host interface units and a plurality of the disk interface units to the shared memory units are not distributed evenly among the shared memory units. On a memory connection path to a shared memory unit into which access requests have been concentrated, the access requests should wait at the switch (interconnection network) on the path. On the other hand, until an access request is generated, a memory connection path to a shared memory unit, to which no access request has been sent, will not operate. Consequently, there develop memory connection paths that do not operate simultaneously at a given time, which has become a factor in limiting the throughput of the internal paths in the starnet connection disk array.
In order to address the technical problem described above, in the starnet connection disk array, there is provided a proposal that the number of connection paths between the host interface units and the switch and between the disk interface units and the switch is made to be greater than the number of the memory connection paths between the switch and the shared memory units. According to this method, a proportion of the memory connection paths between the switch and the shared memory units, which will be used simultaneously is increased. However, the number of the connection paths that can be used simultaneously is determined by the number of the memory connection paths between the switch and the shared memory units. Thus, the effect of improving the maximum throughput of the system would be low. Accordingly, the challenge to be addressed is to increase the number of the memory connection paths between the switch and the shared memory units, which will operate simultaneously, without unnecessarily increasing the number of the connection paths between the host interface units and the switch and between the disk interface units and the switch.
With the increased use of open systems in recent years, it is strongly required that even a disk array support multi-platforms. In other words, the demand for supporting both a high-speed I/F and a low-speed I/F on a single disk array system is keen. The high-speed I/F is represented by a fiber channel, and its throughput reaches 100 MB/s, while the low-speed I/F such as a main frame channel and a Small Computer System Interface (SCSI) channel has its throughput of approximately several tens of MB/s. These must be developed simultaneously in a short period. For this purpose, it is effective that new development of a host interface board for old I/F, the decline of which is expected in the future, is not performed, and that the old model of host interface board can be used as it is without affecting the system performance. Normally, when the starnet connection disk array is made into a new model, the transfer throughput of the connection paths is often improved as means for improving the system performance. In order to effect the above-mentioned purpose, when using the old host interface board, it becomes necessary that the transfer throughput of memory connection paths connecting the switch and the shared memory units is improved while setting the physical I/F and the operating frequency of paths connecting the host interface units to the switch, and connecting disk interface units to switch, to the values of the old model. Hence, the challenge is that the paths connecting the host interface units and the disk interface units to the switch and the memory connection paths connecting the switch to the shared memory units can be operated with their transfer throughputs being different to each other.